The arena's gravity shifts according to a rotating circle on the wall with one arrow on it. When the arrow is pointing straight up, the gravity pulls upwards, and when the arrow points straight down, the gravity pulls downwards. I'm pretty sure the orbs don't control Rinzler, either. It's possible that Clu had a hunch that Sam was a user (being UNKNOWN and all), and was using the orbs as a stress reliever.